1. Field of the Invention
This invention relates to electrical converters and, in particular, to unipolar to bipolar converters. Accordingly, it is a general object of this invention to provide new and improved converters of such character.
2. Description of the Prior Art
Bipolar signals are suitable for use in electrical communication. A common form of bipolar data signals consists of return-to-zero pulses wherein each pulse represents a binary one. The binary one pulses alternate in polarity. A binary zero is represented by the absence of a pulse. A system of return-to-zero pulses is advantageous because timing signals can be simply recovered. The alternating pulse feature is desirable for two reasons: First, the data signal contains no dc component and can be ac coupled with no degradation or loss of energy. Second, the alternating pulse feature provides for automatic error detection. Thus, bipolar transmission is in common use in the electrical telecommunications industry.
In optical communication systems, unipolar transmission is far more desirable than bipolar. In optical communication systems, a binary zero is represented by the absence of light, whereas a binary one is represented by the presence of a light pulse. In present systems, when optical transmission links are used to carry telecommunication signals, the bipolar data signals are converted to unipolar form, transmitted over the optical link, then converted back to bipolar form. Conversion from bipolar to unipolar is relatively simple, while the reverse is usually more difficult.
One method, in the prior art, for converting unipolar pulses to bipolar pulses was to simply apply the unipolar signal to a binary divider which changes state each time a unipolar pulse is present. As the binary divider toggles, the unipolar pulses are alternately inverted, producing a bipolar signal. The major problem with such a prior art method was that very narrow noise pulses, which often occur at the leading and trailing edges of the unipolar pulse, tend to cause the binary divider to toggle twice per unipolar pulse, instead of once. By toggling twice, the binary divider returns to its original position, and the data pulses do not alternate in polarity.
To eliminate false triggering of the binary divider by narrow noise pulses, a timing signal can be derived from the unipolar pulse stream which can be used to sample the pulse stream. Each sample pulse causes the binary divider to toggle as before. However, narrow noise spikes occurring at the leading and trailing edges of the unipolar pulses are eliminated, preventing multiple triggering of the binary divider. Such a method is effective, but requires clock recovery circuits which might not otherwise be required.